Many applications exist for devices which modulate a light beam, e.g. by altering the amplitude, frequency or phase of the light. An example of such a device is a reflective deformable grating light modulator 10, as illustrated in FIG. 1. This modulator 10 was proposed by Bloom et al., in U.S. Pat. No. 5,311,360. The modulator 10 includes a plurality of equally spaced apart, deformable reflective ribbons 18 which are suspended above a substrate 16 having reflective surface portions. An insulating layer 11 is deposited on the silicon substrate 16. This is followed by the deposition of a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14. The nitride film 14 is patterned to form the ribbons and portions of the silicon dioxide layer 12 are etched such that the ribbons 18 are held by a nitride frame 20 on an oxide spacer layer 12. For modulating light having a single wavelength .lambda..sub.0, the modulator is designed such that the thickness of the ribbons 18 and the thickness of the oxide spacer 12 both equal .lambda..sub.0 /4.
The grating amplitude of this modulator 10, defined as the perpendicular distance, d, between the reflective surfaces 22 on the ribbons 18 and the reflective surfaces of the substrate 16, is controlled by applying voltage between the ribbons 18 (the reflective surface 22 of the ribbons 16 serves as a first electrode) and the substrate 16 (a conductive film 24 beneath the substrate 16 serves as a second electrode). In its undeformed state, with no voltage applied, the grating amplitude equals .lambda..sub.0 /2 and the total path length difference between light reflected from the ribbons and the substrate equals .lambda..sub.0, resulting in these reflections adding in phase. Thus, in the undeformed state, the modulator 10 reflects light as a flat mirror. The undeformed state is illustrated in FIG. 2 with incident and reflected light indicated as 26.
When an appropriate voltage is applied between the ribbons 18 and the substrate 16, an electrostatic force deforms the ribbons 18 into a down position in contact with the surface of the substrate 16. In the down position, the grating amplitude is changed to equal .lambda..sub.0 /4. The total path length difference is one-half the wavelength, resulting in the reflections from the surface of the deformed ribbons 18 and the reflections from the substrate 16 interfering destructively. As a result of this interference the modulator diffracts the incident light 26. The deformed state is illustrated in FIG. 3 with the diffracted light in the +/-1 diffraction modes (D.sub.+1, D.sub.-1) indicated as 28 and 30, respectively.
Adhesion between the ribbons 18 and the substrate 16 during wet processing utilized to create the space below the ribbons 18 and during operation of the modulator 10 has been found to be a problem in these devices. Numerous techniques to reduce adhesion have been proposed, including: freeze-drying, dry etching of a photoresist-acetone sacrificial layer, OTS monolayer treatments, use of stiffer ribbons by using shorter ribbons and/or tenser nitride films, roughening or corrugating one or both of the surfaces, forming inverted rails on the underneath of the ribbons, and changing the chemical nature of the surfaces. Sandejas et al. in "Surface Microfabrication of Deformable Grating Light Valves for High Resolution Displays" and Apte et al. in "Grating Light Valves for High Resolution Displays", Solid State Sensors and Actuators Workshop, Hilton Head Island, S.C. (June 1994), have demonstrated that such adhesion may be prevented by reducing the area of contact by forming inverted rails on the underneath of the bridges and by using rough polysilicon films, respectively.
Furthermore, as Apte et al. recognize, a feature of the mechanical operation of the modulator 10 is hysteresis in the deformation of the ribbons 18 as a function of applied voltage. The theorized reason for the hysteresis is that the electrostatic attractive force between the ribbons 18 and the substrate 16 is a non-linear function of the amount of deformation, while the restoring force caused by stiffness and tension of the ribbons 18 is a substantially linear function. FIG. 4 illustrates a simulated hysteresis characteristic where the light output (an indirect indicator of the amount of deformation of the ribbons 18) is shown on the vertical axis and the voltage between the ribbons 18 and the substrate 16 is shown on the horizontal axis. Thus, when the ribbons 18 are deformed into the down position in contact with the substrate 16, they latch in place, requiring a smaller holding voltage than the original applied voltage.
Bloom et al., in U.S. Pat. No. 5,311,360 teach that this latching feature is desirable as it gives the modulator 10 the advantages of active matrix design without the need for active components. In addition, Bloom et al. teach that this latching feature is also desirable in low power applications where efficient use of available power is very important. Recognizing the adhesion problem, however, Bloom et al., teach adding small ridges below the ribbons 18 to reduce the contact area and thereby reduce the adhesion problem. Because the substrate of the modulator 10 is used as an optical surface, however, the manufacturing processes for adding small ridges to the surface is complicated by the requirements that the reflecting portions of the substrate 16 be smooth with high reflectivity and be in a plane parallel to the ribbons 18.
Conventional displays are formed in two dimensional arrays of pixels. The discrete image formed by each of the myriad of pixels are integrated by the eye of the user to form a composite of the pixels representing an overall image. Unfortunately, the cost of such a display system increases because as each pixel is replicated to form the entire array the cost of making each pixel is similarly replicated. Examples of such pixellated displays are televisions and computer monitors. The pixels for each can be formed of LCD devices, or by a CRT.
Therefore, what is needed is a diffraction grating light valve in which adhesion between reflective elements and a substrate is reduced or eliminated without resorting to complicated surface treatments required for reducing such adhesion.
What is also needed is a display that lowers the cost of manufacture by reducing the number of pixels required to build the system without lowering the image quality.